In general, as is disclosed in Patent Document 1 (Japanese Patent Laid-Open No. S62-46292) and Patent Document 2 (Japanese Patent Laid-Open No. H2-6784), a fuel assembly used in a pressurized water nuclear reactor has such a structure that several fuel rods are bundled, that is, in general, several fuel assemblies each composed of bundled several fuel rods are loaded in a reactor vessel suspended in a reactor vessel incorporating an inlet nozzle and an outlet nozzle for a coolant. The fuel assembly has an upper nozzle and a lower nozzle which are opposed to and spaced from each other, and which are connected to each other through the intermediary of control rod guide tubes attached thereto with a plurality of support grids. The control rod guide tubes are inserted in a part of cells in each support grid, and the several fuel rods are inserted in the remaining part of cells.
FIGS. 10 to 13 show a specific configuration of the fuel assembly. Referring to FIG. 10, a control rod assembly used in a pressurized water nuclear reactor is composed of a plurality of control rods 51 suspended from a spider 52 adapted to be driven by a control rod drive unit which is not shown. In the control rod assembly as stated above, as shown in FIG. 11, the control rods 51 are driven by the control rod drive unit so that the control rods are inserted and pulled in the control guide tubes 53 within the fuel assembly loaded in the nuclear reactor, deeply and shallowly in order to control the reactivity of the nuclear reactor core.
However, the control rods 51 are used, being inserted in the control guide tubes 53 within the fuel assembly while they are driven by the control rod drive mechanism during operation of the nuclear reactor, and accordingly, the control rods 51 vibrate due to flows of a coolant during operation of the nuclear reactor so as to make contact with the control guide tubes, possibly resulting in such a risk that the outer surfaces of the control rods are worn. Further, the inner surfaces of the control rod guide tubes 53 would be possibly worn due to the vibration of the control rods 51.
The above-mentioned abrasions are caused by affection of vibration of the control rods 51 due to turbulence of a coolant flow, that is, the coolant in the flow (a core flow directing upward from the lower side) flows transversely through a gap between the control guide tubes 54 (which will be hereinbelow referred to as “G/T”) which have a role of guiding the control rods when the control rod assembly is driven by the control rod drive mechanism, and an upper core plate 55 (which will be referred to as “UCP”) and a gap between the UCP 55 and the upper nozzle 56 of the fuel assembly located therebelow (refer to FIG. 12). Thus, it is construed that the control rods vibrate due to the transverse flow, and accordingly, the abrasion would be gradually progressed.
It is noted here that in comparison with the control rods 51 which are located on the side which is near to the attachment positions of support pins 57 (two pins arranged, left-right symmetrically) for preventing transverse displacement of the G/T 54, the control rods 51 on the side where no support pins 57 are present seem to be greatly affected by the coolant flow. Thus, it is considered that those of the control rods located on the side where no support pins 57 are present are locally worn by a large degree. Further, there would be such a risk that the associated control rod guide tubes 53 in which those of the control rods 51 are inserted are worn at their inner surfaces on the fuel assembly side.
As shown in FIG. 13, it would be also considered that the longitudinal directions of passage holes formed in an adapter plate 58 which constitutes the lower part of the upper nozzle 56 of the fuel assembly greatly affect upon the degree of abrasion. That is, the passage holes 58A are all formed in one and the same direction (the longitudinal direction of the passage hole 58A is indicated by the arrow B as shown in FIG. 13), and accordingly, the flow of the coolant flowing upward from the bottom of the fuel assembly (in the direction piercing the sheet of FIG. 3 from the rear surface to the front surface) passes through the passage holes 58A in the upper nozzle 56, and is then jetted into the upper nozzle 56, being guided by the shapes of the passage holes. The area of the passage holes is greater in the vicinity of the walls on the sides A, that is, on the support pin 57 sides, than in the vicinity of the walls on the sides B where no support pins 57 are present, which are adjacent to the sides A, and accordingly, the quantity of the jetted coolant becomes remarkably greater on the sides A. Since the flows of the coolant impinge upon an overhang formed at the upper end of the upper nozzle 56, and are then directed toward the center of the adapter plate 58, the degree of jetting is different between the sides A and the sides B, resulting in occurrence of such a risk that the flows toward the center of the adapter become unbalance.
Further, due to instability caused by the unbalance flows of coolant toward the center of the adapter 56, there would be caused such a risk that the associated control guide tubes in which the control rods 51 are inserted are greatly worn at their inner surfaces on the fuel assembly side. Thus, there would be presented such a problem that the cause of accelerating the abrasion as stated above is duplicated in such a case that the direction in which no support pines are located coincides with the direction of the passage holes 58A in the upper nozzle 56.
Thus, Patent Document 3 (Japanese Patent Laid-Open No. 2003-98285) discloses a configuration in which the passage holes formed in the adapter plate constituting the lower structure of the upper nozzle are arranged such that the direction of the passage holes is orthogonal to the direction of the sides where there are presented no support pins attached to the control guide tubes and inserted in the upper core plate for preventing the control guide tubes from being transversely shifted. Further, Patent Document 3 also discloses a configuration of the adapter plate constituting the lower structure of the upper nozzle, in which the number of passage holes having a longitudinal direction along the sides where no support pins are present, is decreased with respect to the total number of them, and a configuration in which the passage holes are arranged in the passage surface of the adapter plate so that the passage areas in four zones partitioned by two orthogonal lines passing through the center and diagonal corners of the passage surface of the adapter plate become uniform. With this configuration, the vibration caused by the coolant flow can be uniformed so as to restrain local abrasion.
As stated above, since the control rods in the conventional fuel assembly vibrate due to the transverse coolant flow, there would be possibly caused such a problem that the outer surface of the control rods and the inner surfaces of the control rod guide tubes are worn.
Although the configuration disclosed in Patent Document 3 may more or less improve such abrasion, the present invention proposes such a technology that the vibration of the control rods is further settled in order to minimize the abrasion.